The present invention relates generally to a video printer, and is more specifically directed to the construction of a frame memory and how to use it.
Image recording apparatus so far known to make variable-contrast and multicolor prints of NTSC or other standard formats of TV images include a color video printer, one typical construction of which is illustrated in FIG. 6.
FIG. 6 is a block diagram showing one typical construction of a heat-sensitive sublimation type of color video printer designed to receive an NTSC signal that is a composite image signal, and three primary color signals R, G and B. The NTSC signal is converted in a decoder 14 to three primary color signals R, G and B, which are in turn converted in an A/D converter 16 to digital image data having given gradations, e.g., 256 gradations, for storage in a frame memory 18. Signals R, G and B sent out of an external image signal device such as a monitor are also entered directly into the A/D converter 16, wherein they are converted to digital image signals for storage in the frame memory 18.
The image data stored in the frame memory 18 are read out, one line at a time, under the control of a control unit 40 for writing into a line memory 22. The image data read out of the line memory 22 are converted in a gradation control unit 24 to yellow Y, magenta M and cyan C, the three primary colors for printing, on the basis of the data read out of a gradation control look-up table 26 (hereinafter LUT for short). At this time, the gradation control unit 24 reads the data for gradation correction out of LUT 26 in accordance with a control signal sent out of a control unit 40 in response to a gradation correction command given by an operator from an input unit 36, so that the R, G and B image data or the Y, M and C image data after conversion can be corrected for gradation, using that data. The image data outputted from the gradation control unit 24 is fed to a thermal head 28 for color image printing.
A printing mechanism unit includes a recording paper 32 for recording an image thereon, as shown in FIG. 7. As illustrated, an ink sheet 30 for transferring ink onto the recording paper 32 is in contact with a platen drum 50, and the thermal head 28 is positioned over the ink sheet 30. The recording paper 32 and ink sheet 30 are driven by a printer driving unit 34 of FIG. 6 in the respective directions shown by arrows. As shogun in FIG. 8, the ink sheet 30 is made up of a base 30a such as a polyester film and a sublimable type of ink 30b deposited onto it. With a selected signal electrode of the thermal head 28 energized and heated, the ink 30b deposited onto the base 30a is transferred onto the recording paper 32. As can be seen from FIGS. 7 and 9, the ink sheet 30 is sequentially formed with regions Y, M and C deposited with three inks Y, M and C. One process of recording a color image is finished by making one image print of region Y, then making one image print of regions M, and finally making one image print of region C. The ink sheet 30, if put in a cassette, for instance, is easy to handle.
Referring to FIG. 6, the R, G and B image data written in the frame memory 18 are read from it under the control of the control unit 40, then converted in a D/A converter 42 to analog signals, and finally converted in an encoder 44 to NTSC signals, which are fed to a CRT 46, whereby the image that is being printed is displayed on the CRT 46.
Referring now to the number of pixels when an NTSC format of image is to be printed, satisfactory results are obtained, if about 640 pixels exist in the horizontal direction (i.e., in a row) and about 480 pixels in the vertical direction (i.e., in a column). Therefore, a 512-pixel thermal head may be used as the thermal head 28 in the conventional video printer, and a memory of, e.g., 1,024 pixels (horizontal).times.512 pixels (vertical) may be used as the frame memory 18. When one image is printed on one recording paper, the control unit 40 writes the image data of that image in the frame memory 18, as illustrated in FIG. 10. After this, the image data of X=0 are transferred to a line memory 22 by 512 pixels, i.e., Y=0 to Y=511. Upon the image data written in the line memory 22 transferred to the thermal head 28, read addresses in the X direction are incremented up to transfer the image data of X=1 to the line memory 22 by 512 pixels, i.e., Y=0 to Y=511. The above operation is repeated until X=639, whereby the image mentioned above is printed. As can be clearly appreciated from what has been described, the main printing scan direction is set in the vertical scan direction of a TV image. This shall hereinafter apply.
A video printer is now required not only to be capable of printing one image on one recording paper, but also to have multi-printing functionality that enables a plurality of images to be printed on one recording paper. When multi-printing is done with a conventional video printer, the resultant images are of considerably inferior quality. This is because the image data of the images to be printed are first subjected to pixel-thinning or other suitable processing to reduce them, then all the image data to be printed are written in the frame memory 18, and they are finally transferred to the line memory 22 for printing. For instance, when 16 images are printed on one recording paper, each image is reduced in size to 1/4 in the horizontal and vertical directions for writing in the frame memory 18, as illustrated in FIG. 11, thus making the resultant image very difficult to view.
In recent years, the thermal head 28 is increasingly required to have high density and, with this, a thermal head having 1,024 elements twice as many as conventional is developed, which will hereinafter be called the double-density head. It may be possible to make a print of higher image quality with the use of such a double-density head. It is now noted, however, that the use of the double-density head does not always result in a print of improved image quality, because the structure of the frame memory used and in what form it is used are much the same as conventional.